This invention relates to a control circuit for an electric motor driven vehicle window. Such a circuit includes switches for controlling the application of electric current to the motor armature in a first or second direction to open or close the window; and these switches are often semiconductor switches such as FETs.
However, the motor armature is inductive and, in operation, stores inductive energy which must be dissipated when the motor is switched off so that it does not harm the semiconductor switches. This is normally accomplished by providing anti-parallel diodes around the semiconductor switches. An anti-parallel diode is a diode in parallel with a device and poled so as to allow current flow in the opposite direction to the normal current flow of the device. FETs inherently contain internal anti-parallel diodes, but such diodes are added in circuit with SCRs and other semiconductor switches. These diodes are often called free-wheeling diodes. An example may be seen in the U.S. patent to Lehnhoff U.S. Pat. No. 4,575,662, issued Mar. 11, 1986. The SCRs of that patent have anti-parallel diodes added, while the FETs have such devices inherently.
Anti-parallel diodes, however, may create a problem if the main power supply is connected with the wrong polarity. In a vehicle, if the battery is removed and reinstalled backwards, the anti-parallel diodes may create a short across the battery, which may lead to damage, in the case of FETs, to the FETs of which they are part. Thus, a power window control circuit with reverse battery protection should include some switching device in each possible reverse current path having no such anti-parallel diode. This is not possible with FETs, since the anti-parallel diodes are internal and inherent in the devices themselves. If FETs and SCRs are combined in a switching arrangement, the latter may be provided without anti-parallel diodes. Such an arrangement, however, must provide alternative means for dissipating the inductive energy of the motor armature windings when the switches are opened and the conducting path is broken.